CN114643280A

CN114643280A - Hot rolling method of niobium-containing austenitic stainless steel section

Info

Publication number: CN114643280A
Application number: CN202210408849.7A
Authority: CN
Inventors: 方旭东; 夏焱; 陈超; 张永亮
Original assignee: Shanxi Taigang Stainless Steel Co Ltd
Current assignee: Shanxi Taigang Stainless Steel Co Ltd
Priority date: 2022-04-19
Filing date: 2022-04-19
Publication date: 2022-06-21
Anticipated expiration: 2042-04-19
Also published as: CN114643280B

Abstract

The invention discloses a hot rolling method of a niobium-containing austenitic stainless steel section, which sequentially comprises the following steps: (1) casting the refined molten steel into a steel ingot with a preset size; (2) heating the steel ingot and preserving heat for a certain time; (3) and cogging and rolling the steel ingot according to the set parameters. The hot rolling method solves the cracking problem of the austenitic stainless steel section containing niobium, improves the product quality and the production efficiency, and reduces the production cost.

Description

Hot rolling method of niobium-containing austenitic stainless steel section

Technical Field

The invention belongs to the field of steel material processing, and particularly relates to a hot rolling method of niobium-containing austenitic stainless steel.

Background

In order to improve the power generation efficiency and reduce the emission, the steam temperature and pressure parameters of the coal-fired power plant are continuously improved, the traditional materials cannot meet the requirements of a unit boiler, and a large amount of high-performance niobium-containing austenitic stainless steel materials are needed. The addition of the Nb element can separate out nanometer-sized MX and NbCrN in the service process, and the separated-out MX and NbCrN can be well dispersed and distributed in a matrix to block dislocation movement, improve the precipitation strengthening effect and creep resistance and improve the high-temperature resistance of the austenitic stainless steel. However, because the Cr and Ni contents of the material are higher, massive niobium compounds are easy to form in the solidification process, the thermoplasticity of the material is reduced, and the material is easy to crack in the hot rolling process. Compared with the conventional process of 'steel ingot blooming and hot rolling', the stainless steel processed by the process has the advantages of uneven internal structure, low production efficiency and high manufacturing cost.

Disclosure of Invention

In view of the above problems, an object of the present invention is to provide a hot rolling method of an austenitic stainless steel profile containing niobium.

Specifically, the invention is realized by the following technical scheme:

a hot rolling method of niobium-containing austenitic stainless steel sectional materials sequentially comprises the following steps:

(1) casting the refined molten steel into a steel ingot;

(2) heating and preserving heat of the steel ingot;

(3) cogging and rolling the steel ingot;

wherein, in step (1), the steel ingot has the following dimensions:

the maximum circumscribed circle diameter or diagonal Dmax of the section of the steel ingot is less than or equal to 440mm,

the minimum circumscribed circle diameter or diagonal Dmin of the section of the steel ingot is less than or equal to 420mm,

the height H of the steel ingot is more than 1500mm,

the taper (Dmax-Dmin)/H of the steel ingot is 0.3-3%.

Optionally, the steel ingot is a steel ingot with a head volume ratio of less than or equal to 10% or a steel ingot without a head.

Optionally, in step (1), the refining molten steel comprises, in weight percent: 0.04 to 0.10 percent of C, less than or equal to 0.75 percent of Si, less than or equal to 2.00 percent of Mn, less than 0.03 percent of P, less than 0.03 percent of S, 24.0 to 26.0 percent of Cr, 17.0 to 23.0 percent of Ni, 0.20 to 0.60 percent of Nb, 0.15 to 0.35 percent of N, and the balance of Fe and inevitable impurities.

Optionally, in step (2), the target temperature for heating the steel ingot is 1250 ℃ to 1270 ℃, and the holding time T ═ 0.8 × Dmax + T1 × w (nb) × 100) minutes, wherein:

dmax is the maximum circumscribed circle diameter or diagonal of the section of the steel ingot, and the unit is millimeter,

t1 is the heat-compensating coefficient, which ranges from 100 to 300,

w (Nb) is the weight percent of the element Nb.

Optionally, in the step (3), the rolling total compression ratio is not less than 4, the rolling reduction of a single pass is not more than 100mm, and the width-height ratio of a vertically rolled piece is not more than 3.

Optionally, in the step (3), the rolling surface temperature is 1100-1150 ℃, and the rolling process and the finish rolling surface temperature are more than or equal to 900 ℃.

The austenitic stainless steel section containing niobium is prepared by the hot rolling method.

Optionally, the niobium-containing austenitic stainless steel profile is a billet or bar.

Optionally, the niobium-containing austenitic stainless steel profile comprises, in weight percent: 0.04 to 0.10 percent of C, less than or equal to 0.75 percent of Si, less than or equal to 2.00 percent of Mn, less than 0.03 percent of P, less than 0.03 percent of S, 24.0 to 26.0 percent of Cr, 17.0 to 23.0 percent of Ni, 0.20 to 0.60 percent of Nb, 0.15 to 0.35 percent of N, and the balance of Fe and inevitable impurities.

Compared with the conventional multi-fire-diameter forging process, the niobium-containing austenitic stainless steel profile produced by the hot rolling method has the following advantages:

the uniformity of the structure is good, and the grain size is 4-7 grades;

the hot perforation performance is better, and the internal cracking rate is within 5 percent;

the method has higher billet yield, the billet yield of the forged billet is 65 percent, and the billet yield of the rolled billet can be improved to more than 85 percent.

Drawings

FIG. 1 shows the columnar structure of the steel ingot of example 1 in the original state, x 100 times;

FIG. 2 shows the columnar structure of the ingot, x 100 times, after heating of the ingot of example 1;

FIG. 3 shows the quality of the surface of the billet during the rolling of the billet in example 1;

FIG. 4 shows the quality of the surface of the billet after rolling in example 1;

FIG. 5 shows the texture uniformity of the square billet obtained in example 1;

FIG. 6 shows the columnar structure of the steel ingot, x 100 times, after the steel ingot of comparative example 1 is heated;

FIG. 7 shows the quality of the billet surface during rolling of a billet in comparative example 1;

fig. 8 shows the quality of the surface of the rolled billet of comparative example 1.

Detailed Description

The present invention will be described in detail with reference to the following embodiments in order to fully understand the objects, features and effects of the invention. The process of the present invention employs conventional methods or apparatus in the art, except as described below. The following noun terms have meanings commonly understood by those skilled in the art unless otherwise specified.

Aiming at the defects of the existing hot processing method of the niobium-containing austenitic heat-resistant stainless steel bar, the invention optimizes the production process, selects a steel ingot with special size, controls the hot rolling key process parameters, adopts the processes of steel ingot blooming and cogging and hot rolling on the basis of ensuring the material structure performance, solves the problem of material hot rolling cracking, realizes the technological progress of replacing hot forging with hot rolling, improves the product quality, improves the production efficiency and reduces the manufacturing cost.

In the present invention, "niobium-containing austenite", "niobium-containing austenitic stainless steel", "niobium-containing austenitic heat-resistant stainless steel", "niobium-containing austenitic stainless steel profile", "niobium-containing austenitic heat-resistant stainless steel profile", and the like have the same meanings and may be used interchangeably, and the chemical compositions thereof are as follows (in weight%):

the characteristics of the niobium-containing austenitic stainless steel are as follows: the alloy content is up to more than 45 percent, and the alloy also contains higher N content, so that the high-temperature deformation resistance is large, and the deformation resistance at 1100-1200 ℃ is 1.5-2 times that of the traditional austenitic stainless steel; high Cr and Nb content easily form large-block niobium compounds at Nb element segregation positions during alloy solidification, and cause local molding deterioration and local cracks.

The main technological measures of the hot rolling of the austenitic stainless steel pipe blank containing niobium of the invention comprise: casting steel ingots with special size requirements, reasonable heating process parameters, cogging rolling process parameters and the like.

In the aspect of process flow design, because the thermoplasticity of the material in an as-cast structure is extremely poor, if the process parameters of heating and rolling cogging of steel ingots with conventional sizes are adopted, the rolling direction metal is tensile stress deformation, the deformation condition is poor, the whole plasticity of the metal cannot be fully exerted, and surface cracking is caused. However, the conventional steel ingot is adopted, a multi-fire forging process is required, on one hand, the metal mainly bears compressive stress in the deformation process, the plasticity is good, a deformation recrystallization structure is formed after forging, the thermoplasticity of the material is improved, and the economical efficiency of the product is seriously influenced due to high forging cost and low production efficiency. From the perspective of reducing the manufacturing cost and developing the efficiency of equipment, the process of 'steel ingot blooming and hot rolling' is selected on the overall process design. The thermoplasticity and the economical efficiency of the niobium-containing austenitic stainless steel are improved by optimally designing the size of the steel ingot and the heating process parameters. By optimizing the rolling process parameters, the smooth rolling is ensured, and the tissue uniformity of the material is improved.

The total process route of niobium-containing austenite hot working is as follows: special-sized steel ingot → steel ingot heating → cogging rolling → inspection → delivery.

As a preferred embodiment, the hot rolling method of the niobium-containing austenitic stainless steel profile of the present invention comprises the steps of:

(1) and casting the refined molten steel into a steel ingot.

And casting the refined molten steel meeting the requirements of chemical components and purity into steel ingots with special sizes. The refining of molten steel can be carried out according to the scheme in the prior art, and a person skilled in the art can select a proper method according to production needs, which is not described herein.

The special dimensions of the steel ingot are as follows:

the maximum circumscribed circle diameter of the section of the steel ingot or the diagonal Dmax of the maximum section of the steel ingot is less than or equal to 440 mm;

the diameter of the minimum circumscribed circle of the section of the steel ingot or the diagonal Dmin of the minimum section of the steel ingot is less than or equal to 420 mm;

Dmin＜Dmax；

the height H of the steel ingot is more than 1500 mm;

the taper (Dmax-Dmin)/H of the steel ingot is 0.3 to 3 percent;

the steel ingot is a steel ingot with a head volume ratio of less than or equal to 10 percent or a steel ingot without a head.

In order to improve the hot workability of the material, the steel ingot with special size requirements is designed and selected as a hot rolling raw material on the basis of the production of the conventional steel ingot. The method is characterized in that a conventional steel ingot with the single weight of more than 5 tons and large section size is selected, in the solidification process of molten steel, under the action of selective crystallization, serious component segregation of Cr, Ni, Nb and other alloy elements is formed between the head and the tail of the steel ingot and solidified dendrite trunk and dendrite liquid, the section size of the steel ingot is large, an as-cast structure after solidification is not compact, serious looseness or shrinkage cavities often exist along the axial direction of the steel ingot, and the aggregation and interweaving conditions of initial Nb, such as NbCrN, MX and other niobium compounds formed at the solidification front are accompanied. The uneven structure due to solidification makes it difficult to improve the original structure thermoplasticity and the structure uniformity after rolling even when hot rolling is performed at a large reduction ratio. The design selects the conventional steel ingot with the single weight less than 2 tons and the small section size, the casting process is long, the superheat degree difference between every two plates of the cast steel ingot is large, the casting speed needs to be adjusted in time, the quality fluctuation of the cast steel ingot of different plates is large, the difficulty in improving the rolling quality through the hot rolling compression ratio is high due to the small section, the production efficiency is low, and the manufacturing cost is high. The inventor of the invention carries out a large number of experiments, and determines the special size steel ingot special for producing the niobium-containing austenitic stainless steel section through continuously optimizing and designing the size of the steel ingot and carrying out practice for many times. The method specifically comprises the following steps:

the hot rolling raw material of the niobium-containing austenitic stainless steel section can be a square ingot, a rectangular ingot, a round ingot or an octagonal ingot. For the austenitic stainless steel containing niobium with high alloy content, a round ingot or an octagonal ingot which is uniformly cooled in the radial direction in the solidification process is preferably selected.

The maximum circumscribed circle diameter of the section of the steel ingot or the diagonal Dmax of the maximum section of the steel ingot is less than or equal to 440mm, the minimum circumscribed circle diameter of the section of the steel ingot or the diagonal 290mm of the minimum section of the steel ingot is less than or equal to Dmin and less than or equal to 420mm, the diameter size of the circumscribed circle of the section of the steel ingot is limited to [440 mm-290 mm ], and the quality problems of serious segregation and shrinkage cavities in the molten steel solidification process, the gathering of niobium compounds such as initial NbCrN, MX and the like in the center of the steel ingot and the like caused by overlarge section size of an ingot mold in the casting process can be prevented, or the defects can be improved to the extent that the section size of the inner wall of the hot perforation is not influenced after the subsequent hot rolling process.

In the present invention, the height of the ingot means the maximum distance from the bottom of the ingot to the head of the ingot or the maximum dimension of the linear distance of the ingot in a certain direction, and the section of the ingot means the cross section perpendicular to the height direction of the ingot, which should be the cross section perpendicular to the height direction.

The taper (Dmax-Dmin)/H of the steel ingot is as follows: 0.3-3% and the height H of the steel ingot is more than 1500 mm. The conicity of the steel ingot is too large, and the rolling mill is difficult to cogging or folds and the like are generated in rolling. The taper is too small, which can affect the solidification process of molten steel in the ingot mould and the internal quality of steel ingots. In addition, the height of the steel ingot can be maximized in terms of improving the yield of the steel ingot, as allowed by a cast-in-place system.

The steel ingot with the head-to-volume ratio less than or equal to 10 percent is designed into a small feeder head or a steel ingot without the feeder head, and the molten steel feeding in the solidification process of the niobium-containing austenitic stainless steel can be promoted by adopting heat preservation measures such as a heat preservation cap, a soaking plate, a heating agent and the like, so that the tissue compactness of the feeder head and the nearby area thereof is improved, and the steel ingot blank forming rate is improved. The casting temperature of molten steel is controlled well by using a non-riser steel ingot, the volume shrinkage in the solidification and cooling processes is reasonably utilized, and the steel ingot blank forming rate is improved to the maximum extent.

(2) And heating and preserving heat of the steel ingot.

The target heating temperature for heating the steel ingot is 1250-1270 ℃, and the heat preservation time T is 0.8 multiplied by Dmax + T1 multiplied by w (Nb) multiplied by 100, the unit of the heat preservation time T is minutes, wherein:

dmax is the maximum circumscribed circle diameter of the section of the steel ingot or the diagonal line of the maximum section of the steel ingot, and the unit is millimeter;

t1 is the heat compensation coefficient, the range is 100-300, when the Dmax is large, t1 takes the upper limit value, otherwise takes the lower limit value, the concrete is as follows:

Dmax	t1
		Dmax＝440mm	300
400≤Dmax＜440mm	240≤t1＜300
		360≤Dmax＜400mm	180≤t1＜240
Dmax＜360mm	100≤t1＜180

w (Nb) is the weight percent of the element Nb.

By analyzing the phase diagram of the niobium-containing austenitic stainless steel and observing the phase diagram with experiments, the material is heated at 1270 ℃ or higher, a low-melting-point eutectic phase of niobium appears, and an NbC phase begins to be precipitated below 1200 ℃, and the NbC phase precipitated in a steel ingot is often in a massive state due to segregation of Nb elements in solidification. Therefore, the target heating temperature is 1250-1270 ℃, so that NbC can be prevented from being precipitated to influence thermoplasticity, and low-melting-point eutectic phase can be prevented from being generated at high temperature.

According to the traditional heating system, the heat preservation time after the temperature is up to the temperature is 0.5-0.8 min/mm according to the difference of the diameter D of the steel ingot, and the steel ingot can be thoroughly heated in the time, but NbC cannot be dissolved. In order to eliminate the massive niobium compounds such as NbCrN, MX and the like formed in the solidification process as much as possible, the holding time should be properly prolonged. However, after the holding time is too long, the crystal grains are easily coarse, and grain boundary cracks are easily formed.

Through a large number of experimental researches, the inventor obtains the heat preservation time formula, so that NbC can be dissolved while the hot penetration of the steel ingot is ensured, and the coarseness and the grain boundary cracks of crystal grains are avoided while large-block niobium compounds such as NbCrN, MX and the like formed in the solidification process are eliminated.

(3) Cogging rolling

The total compression ratio is more than or equal to 4, the initial rolling surface temperature is 1100-1150 ℃, the rolling process and final rolling surface temperature is more than or equal to 900 ℃, and the rolling single-pass reduction is less than or equal to 100 mm.

The total compression ratio of cogging rolling is more than or equal to 4. The rolling reduction of a single pass is less than or equal to 100 mm. By controlling the total deformation and single-pass reduction in the cogging process, the as-cast structure of the niobium-containing austenitic stainless steel ingot is fully deformed to form a fiber structure after thermal processing, the consistency of the microstructure and the performance of the center, the radius 1/2 and the surface of a rolled piece is improved, and the internal quality of the material is improved.

The surface temperature of a rolled piece rolled at the beginning in the rolling process is controlled to be 1100-1150 ℃, the surface temperature of the rolled piece rolled at the end is controlled to be more than or equal to 900 ℃, the metal can be ensured to be quickly deformed in the optimal thermoplastic temperature range and under the lower deformation resistance in the deformation process, the dynamic recrystallization in the deformation process of the deformed metal is more sufficient, and the condition of difficult rolling or cracking in rolling is prevented.

In the deformation process, according to the specification of a finished product, a proper pass and a proper deformation pass are selected, the width-to-height ratio of a rolled piece to be rolled is less than or equal to 3, a good matching relation between the material shape size and the pass of the rolled piece in the deformation process is ensured, and the rolling stability is ensured while good appearance size and surface quality are obtained.

After the rolling is finished, the inspection is carried out according to the conventional operation, and the detailed description is omitted.

Examples

The invention is further illustrated by the following examples, which are not intended to limit the invention thereto. The experimental methods without specifying specific conditions in the following examples were selected according to the conventional methods and conditions, or according to the commercial instructions.

Example 1:

the niobium-containing austenitic stainless steel of the present example had the following elemental composition (wt%):

composition (I)	C	Si	Mn	P	S	Cr	Ni	Nb	N
										Content (wt.)	0.05	0.21	0.76	0.024	0.001	25.1	20.3	0.40	0.26

In this example, the ingot gauge is Φ 440mm, and the target gauge is 150mm × 150 mm.

The hot rolling process of this example is as follows:

1. ingot casting

Casting the refined molten steel into round ingots, wherein Dmax is 440mm, Dmin is 390mm, H is 1820mm, the taper is 2.7%, and the emission capacity is 9%.

The columnar crystal structure of the steel ingot is shown in figure 1.

2. Heating a steel ingot: the target heating temperature is 1270 ℃, the holding time is 472min, and t1 is 300.

After heating, the ingot columnar grain structure is shown in fig. 2.

3. Cogging and rolling:

the initial rolling temperature is 1140 ℃, and the rolling process and final rolling temperatures are 987 ℃. The single-pass rolling reduction is controlled to be 30-90 mm. Total compression ratio: 6.75. specification of a finished product: the side length is 150mm multiplied by 150mm square billet.

The surface of the billet is not cracked during the rolling process, as shown in figure 3, and the surface quality of the billet after rolling is shown in figure 4.

The structural uniformity of the billet obtained in this example is shown in fig. 5.

Example 2:

composition (I)	C	Si	Mn	P	S	Cr	Ni	Nb	N
										Content (c) of	0.05	0.13	0.87	0.028	0.001	24.9	21.1	0.45	0.22

In this example, the ingot gauge is 400mm Φ, and the target bar gauge is 110mm Φ.

The hot rolling process of this example is as follows:

1. ingot casting

Casting the refined molten steel into round ingots, wherein Dmax is 380mm, Dmin is 362mm, H is 1920mm, the taper is 0.94 percent, and the top-hat ratio is 8 percent.

2. Heating a steel ingot: the target heating temperature is 1250 ℃, the holding time is 400min, and t1 is 220.

3. Cogging and rolling:

the start rolling temperature was 1120 ℃, the rolling process and the finish rolling temperature was 910 ℃. The single-pass rolling reduction is controlled to be 10-70 mm. Total compression ratio: 13.2. specification of a finished product: 110mm diameter bar.

Comparative example 1:

the elemental composition of the niobium-containing austenitic stainless steel of this comparative example was the same as that of example 1.

The hot rolling process of this comparative example is as follows:

1. steel ingot casting: 5.8 tons of square ingot, maximum cross section of steel ingot: 660mm, minimum 580mm, height 1990, taper 0.04, and capacity at break 13%.

2. Heating a steel ingot:

the target heating temperature was 1230 ℃ and the incubation time was 500 minutes (0.8X 660: 528 min, calculated as 0.8 min/mm).

After heating, the ingot columnar grain structure is shown in fig. 6.

3. Forging or rolling:

the surface of the billet cracked during the rolling process as shown in fig. 7, and the surface quality of the billet after rolling as shown in fig. 8.

And (3) performance testing:

the grain size detection method comprises the following steps: GB/T6394-2017 (latest edition)

The method for detecting the internal cracking rate of the hot perforation comprises the following steps: pickling the perforated tubular billet, carrying out visual inspection on the inner wall of the tubular billet by adopting an endoscope method after pickling, and carrying out a statistical method: capillary qualified weight/capillary inspected weight

The finished product rate detection method comprises the following steps: the steel needs to be subjected to GB4162-B flaw detection. Yield is the weight of the section bar passing the flaw detection/the weight of the steel ingot corresponding to the production.

	Example 1	Example 2	Comparative example 1
				Grain size	5-6	5-6	3-5
Hot perforation internal crack rate	98.2％	96.5％	72.3％
				Yield of finished products	85.9％	87.8％	64.6％

As can be seen from the data in the table above, the stainless steel section produced by the hot rolling method of the invention has good structure uniformity, and the hot perforation performance and the billet yield are both obviously improved.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other substitutions, modifications, combinations, changes, simplifications, etc., which are made without departing from the spirit and principle of the present invention, should be construed as equivalents and included in the protection scope of the present invention.

Claims

1. A hot rolling method of niobium-containing austenitic stainless steel sectional materials sequentially comprises the following steps:

(1) casting the refined molten steel into steel ingots;

(2) heating and preserving heat of the steel ingot;

(3) cogging and rolling the steel ingot;

characterized in that, in step (1), the ingot has the following dimensions:

the height H of the steel ingot is more than 1500mm,

the taper (Dmax-Dmin)/H of the steel ingot is 0.3-3%.

2. A hot rolling method according to claim 1, wherein the ingot is an ingot having a head-to-volume ratio of 10% or less or an ingot having no head.

3. A hot rolling method according to claim 1, wherein in the step (1), the refined molten steel comprises, in weight percent: 0.04 to 0.10 percent of C, less than or equal to 0.75 percent of Si, less than or equal to 2.00 percent of Mn, less than 0.03 percent of P, less than 0.03 percent of S, 24.0 to 26.0 percent of Cr, 17.0 to 23.0 percent of Ni, 0.20 to 0.60 percent of Nb, 0.15 to 0.35 percent of N, and the balance of Fe and inevitable impurities.

4. A hot rolling method according to claim 1, wherein in step (2), the target temperature for heating the steel slab is 1250 ℃ to 1270 ℃ and the holding time T ═ time (0.8 x Dmax + T1 x w (nb) x 100) minutes, in which:

t1 is the heat-compensating coefficient, which ranges from 100 to 300,

w (Nb) is the weight percent of the element Nb.

5. A hot rolling method according to claim 1, characterized in that in the step (3), the rolling total reduction ratio is more than or equal to 4, the rolling single-pass reduction is less than or equal to 100mm, and the width-to-height ratio of the immediately rolled piece is less than or equal to 3.

6. A hot rolling process according to claim 1, characterized in that in step (3), the start rolling surface temperature is 1100 ℃ to 1150 ℃, the rolling process and the finish rolling surface temperature are not less than 900 ℃.

7. A niobium-containing austenitic stainless steel profile, characterized by being produced by the hot rolling method according to any one of claims 1 to 5.

8. The niobium-containing austenitic stainless steel profile of claim 7, wherein the niobium-containing austenitic stainless steel profile is a billet or bar.

9. The niobium-containing austenitic stainless steel profile of claim 7, wherein the niobium-containing austenitic stainless steel profile comprises, in weight percent: 0.04 to 0.10 percent of C, less than or equal to 0.75 percent of Si, less than or equal to 2.00 percent of Mn, less than 0.03 percent of P, less than 0.03 percent of S, 24.0 to 26.0 percent of Cr, 17.0 to 23.0 percent of Ni, 0.20 to 0.60 percent of Nb0.15 to 0.35 percent of N, and the balance of Fe and inevitable impurities.